The present invention generally relates to endodontic instruments, such as files and reamers, and more specifically, to those instruments especially useful in root canal procedures.
Endodontists use various types of instruments for cleaning and enlarging the root canals of the teeth. In a typical root canal procedure, an endodontist first makes an opening in the surface of the tooth to provide access to the interior. The endodontist then utilizes small instruments, such as hand-held files and reamers, to clean and enlarge the narrow, tapered root canals. In a conventional procedure, the endodontist fills the prepared root canals with gutta percha, which is a rubber-like substance, and then seals the tooth with protective cement. The endodontists may sometimes apply a crown to the tooth as a final step.
Typically, the endodontist uses a series of delicate, flexible files to clean out and shape the root canals. Each file includes a proximal end, typically including a handle to be gripped between the fingers of the endodontist, and a distal end or tip. A working length with helical or non-helical flutes and cutting edges is located between the proximal and distal ends. The endodontist uses files of increasingly larger diameter to sequentially increase the diameter of the root canal and achieve the desired diameter and shape.
Endodontic instruments of the desired type having helical flutes are conventionally fabricated by permanently twisting a rod of triangular, square, or rhomboid-shaped cross section. The angles formed between the surfaces form the cutting edges which spiral along the working length of the instrument. Another method for manufacturing instruments of the described type having either helical or non-helical flutes is by a machining process wherein an instrument blank is moved past a rotating grinding wheel. The instrument blank is thereafter indexed and again moved past the grinding wheel, and these steps are repeated as many times as are necessary to form the instrument blank into the desired cross section. The flute grinding process, however, produces a directional surface finish along the cutting axis which propagates early material failure, introduces machining stresses into the material, and cuts across the grain structure of the instrument blank. In addition, these direct grinding methods are time consuming and expensive. They also limit the variety of cross-sectional shapes that may be formed in the final product.
Over the past several years, endodontic instruments having helical flutes have been manufactured by simultaneously grinding and twisting thin carbon steel or stainless steel rods or wires. Specifically, steel wire blanks are first ground to the desired cross sectional shape, such as square, triangular or rhomboid, and to the appropriate size and taper. The ground blank is then gripped at one end and spring loaded jaws are brought into contact with the ground portion of the blank. As the blank is rotated from the gripped end, the jaws are moved axially away from that end. The jaws therefore twist the rotating blank and form helical flutes into the blank. The longitudinal, ground edges of the blank form helical cutting edges on the file. The axial jaw speed, twisting speed and spring force are controlled to obtain the desired helical configuration.
Carbon and stainless steel instruments are generally stiff, which may lead to errors during root canal therapy. With the emergence of superelastic materials, such as nickel-titanium alloys, endodontic instrument manufacturers are now able to form endodontic root canal files and reamers with much more flexibility. This greatly assists the endodontist during use of the file or reamer in a root canal procedure. The use of superelastic material, however, causes some significant manufacturing concerns due to the tendency of the material to return to its original shape after the release of an applied force. File or reamer blanks manufactured of superelastic materials generally react in this manner to the conventional twisting methods employed for manufacturing carbon and stainless steel files and reamers. Moreover, if superelastic blanks are over-stressed, such as by being twisted too much during the fluting procedure, the material is subject to failure. For reasons such as these, current manufacturers of endodontic instruments may resort to grinding the helical profile directly into the superelastic blanks while applying no twisting forces to the blanks. These direct grinding methods are time consuming and expensive. They also limit the variety of cross sectional shapes that may be formed in the final product.
In U.S. Pat. No. 6,149,501, a method is provided for manufacturing superelastic endodontic instruments in which a blank is provided and maintained in the austenite phase, preferably above the austenite finish temperature (Af), at least prior to a twisting operation and, preferably, prior to and during the twisting operation. During the twisting operation, the material is converted from the austenite phase to the martensite phase by the stress applied during the twisting operation. Thus, the superelastic material undergoes stress-induced martensite transformation from a 100% austenite phase. For this method, high temperature tooling is required because the twisting operation is performed at a temperature above the Af temperature. The tooling and file blank are preferably submerged in a heated liquid, such as an oil or salt solution at a temperature of 500xc2x0 C. or above, to bring the material to a 100% austenite phase. The heated liquids, however, are generally corrosive to the tooling.
With the above background in mind, there is a need for endodontic instruments, such as files and reamers, and to methods of fabricating such endodontic instruments that avoids the disadvantages described above for grinding and/or twisting techniques, that provides an instrument having either helical or non-helical flutes, and that is flexible and highly resistant to torsional breakage. It would further be desirable to provide a method of manufacturing a wide variety of superelastic endodontic instruments using a twisting technique that does not require high temperature tooling.
The present invention provides a method for forming superelastic endodontic instruments in which helical flutes may be formed in a blank by twisting the instrument blank at low temperature, for example a temperature less than about 100xc2x0 C., and advantageously at ambient temperature. To this end, wire of superelastic material, such as a nickel-titanium alloy wire, is formed into an instrument blank, wherein before twisting, the superelastic material is brought to an annealed state comprising a phase structure that is a rhombohedral phase, a combination of an austenite phase and a martensite phase, a combination of a rhombohedral phase and an austenite phase, a combination of a rhombohedral phase and a martensite phase, or a combination of a rhombohedral phase, an austenite phase and a martensite phase. In this annealed state, the instrument blank is twisted at low temperature to the final twisted configuration desired for the instrument. The twisted instrument is then heat treated, for example at a temperature of at least about 300xc2x0 C., followed immediately by rapid quenching to a superelastic condition. To provide the superelastic material in the annealed state, the material may be annealed at a temperature in the range of about 250-700xc2x0 C., and advantageously at about 350-550xc2x0 C., then cooled to ambient temperature. This annealing may be performed before forming the alloy wire into an instrument blank, or after the instrument blank has been formed. After rapidly quenching the twisted instrument, the method may further comprise a stress relieving heat treatment, for example at a temperature in the range of about 150-300xc2x0 C. for a period of about 2-6 hours.
The present invention further provides a method for manufacturing endodontic instruments having either helical or non-helical flutes, having hard surfaces with a non-directional surface finish and resilient cutting edges, and having both flexibility and resistance to torsional breakage. To this end, flutes are formed in an instrument blank by either EDM or ECM. EDM refers to machining methods including electrical discharge machining, wire electrical discharge machining and electrical discharge grinding. ECM refers to electrochemical machining. Using either an EDM or ECM process, material is removed from the instrument blank in the desired flute pattern with at least about 25% of the diameter of the instrument blank being removed at a point of maximum metal removal. The EDM or ECM process disintegrates the surface material, and as it cools, at least a portion of the removed material re-deposits onto the surface being machined, i.e. onto the flutes being formed, to form a recast layer. The recast layer on the instrument has a surface hardness that is at least about 15% greater, such as about 15-25% greater, than the hardness of the material forming the instrument blank. Thus, surface hardness is increased along the flutes, which provides a significantly harder and more resilient cutting edge for the endodontic instrument. Additionally, the EDM or ECM process produces a non-directional surface finish, thereby avoiding inducement of early material failure propagated by the directional surface finish that results from conventional grinding techniques.
In one exemplary method of the present invention, by rotating the instrument blank about its center longitudinal axis, while advancing the instrument blank past an electrode, helical flutes may be formed. In an alternative exemplary method of the present invention, by holding the instrument blank stationary while advancing an electrode past the instrument blank, non-helical flutes in axial alignment may be formed. There is thus provided a method for efficiently producing precision endodontic instruments with minimal distortion, non-directional surface finish, and with hardened cutting edges along the working length of the instrument in both helical and non-helical flute designs.
The present invention further provides a method in which the instrument blank is formed by EDM or ECM, followed by twisting the blank at low temperature to form helical flutes, thereby providing a low temperature twisting method with the benefits of surface finish and hardness achieved by EDM and ECM.